Field of the Invention
The present invention relates to a technique for enhancing the in-plane uniformity of optical characteristics of an optical film formed in a photomask blank.
Description of the Related Art
Along with higher integration of semiconductor integrated circuits, further refinement and higher precision are required for patterns formed on photomasks. Consequently, for the reason of an enhancement in resolution, exposure light having a shorter wavelength is increasingly used for a photolithography technique. Specifically, the used exposure light is being shifted from g rays (a wavelength λ=436 nm) and i rays (λ=365 nm), which are emitted from ultraviolet light sources, to KrF rays (λ=248 nm) and ArF rays (λ=193 nm), which are emitted from far-ultraviolet light sources.
Further, in order to achieve such further refinement and higher precision of patterns, liquid immersion technology, resolution enhancement technology (RET), and technology such as double exposure (double patterning lithography) are also applied.
Meanwhile, as the wavelength of exposure light is shorter, the depth of focus (DOF) is narrower. Hence, a focus error is more likely to occur at the time of pattern transfer, and this causes a decrease in manufacture yield.
One of the methods of improving such a depth of focus is a phase shift method. According to the phase shift method, patterns are formed using a phase shift mask such that the phases of patterns adjacent to each other are different by approximately 180°. According to the phase shift method, light that has passed through a region in which a phase shift film is formed and light that has passed through a region in which a phase shift film does not exist interfere with each other in a boundary portion therebetween, and the obtained light intensity distribution exhibits a sudden change in the boundary portion, so that the image contrast is enhanced. Note that the phase shift mask includes masks of Levenson type and halftone type.
A phase shift mask including a phase shift film made of molybdenum silicide oxide (MoSiO) or molybdenum silicide oxide nitride (MoSiON) has been proposed as the halftone phase shift mask (see, for example, Japanese Patent Laid-Open No. 7-140635). Further, a phase shift film containing chromium and a phase shift film containing tantalum have also been proposed as the halftone phase shift mask (Japanese Patent Laid-Open Nos. 9-244212 and 2001-312043).
Such a phase shift mask is formed by pattern-forming a phase shift mask blank according to a lithography method. For example, in the case of the halftone phase shift mask, a photomask blank obtained by forming a halftone phase shift film and a light shielding film (such as a Cr film) in order on a transparent substrate is used.
A resist is applied onto this photomask blank, and a desired portion of the resist is exposed to electron rays or ultraviolet rays and is then developed, whereby the resist is patterned. Then, the light shielding film and the phase shift film are removed by etching using the patterned resist film as a mask. Then, if the resist film and the light shielding film are peeled off, the halftone phase shift mask is obtained as a result.
Note that a plurality of photomasks are used to manufacture a device having a structure in which a plurality of layers are laminated, and superimposition with high precision is required in this case. Further, double patterning, which is a new technique of performing exposure to light with a layout being divided into two (or more) photomasks, is required to achieve superimposition with still higher precision.
In the case where a thin film formed on a substrate is already stressed in a photomask blank state, unfortunately, the film stress is partially released in the respective steps of resist application, exposure to light, development, etching, and resist peel-off for pattern formation, and the finally obtained photomask itself is “distorted”. Such distortion makes the precision of photomask superimposition lower, and causes defects.
The level of such “distortion” depends on a formed pattern and the magnitude of a film stress, and it is extremely difficult to control the level of such “distortion” during a photomask manufacturing process.
If each thin film is formed under such conditions that can make the stress in the thin film approximately zero, this problem does not occur, but it is extremely difficult and practically impossible to find manufacturing process conditions that can satisfy both: film formation conditions for securing characteristics to be provided to the thin film as an optical film; and conditions for forming a low-stress thin film. For this reason, it is necessary to provide, as different independent steps, a step of forming the thin film under conditions that can secure the characteristics of the thin film and a step of reducing the stress in the thin film.
In general, a thin film such as a phase shift film in a photomask blank is formed according to a sputtering method. A stress is generated in the thin film in the course of the film formation process, and this stress distorts the substrate itself, and warps the photomask blank.
The following technique has been proposed as a solution to this problem. That is, the film stress is controlled by irradiating the light absorbing thin film such as a phase shift film with light from a flash lamp at a predetermined energy density, whereby the warpage of the photomask blank is reduced (Japanese Patent Laid-Open No. 2004-199035).
It is considered that, if a light absorbing film such as a halftone phase shift film is irradiated with flash light, absorption of the irradiation light, a rapid change in temperature, and the like change the film composition, the atom bonding state, and the like, to thereby change optical characteristics of the film and relieve the film stress.
A hot plate, a heater, a halogen lamp, an infrared lamp, and a furnace are conceivable as means for externally imparting energy for stress relief of a thin film. According to these means, the substrate itself is damaged by an increase in substrate temperature caused by imparting the energy, and the productivity is decreased by an increase in processing time. Hence, such light irradiation using a flash lamp as described in Japanese Patent Laid-Open No. 2004-199035 is superior.
Although the stress is improved by light irradiation using a flash lamp, the amount of absorbed light is different between a peripheral region and a central region of an optical film formed on a substrate to be irradiated, depending on the structure of a flash lamp irradiation apparatus and the shape of the substrate, so that optical characteristics of the optical film fluctuate in the same plane. A technique for improving this is disclosed in Japanese Patent Laid-Open No. 2007-114681.
A roadmap concerning lithography of ITRS in 2011, however, requires the in-plane uniformity of optical film characteristics to be higher.
The present invention, which has been made in view of such a requirement, has an object to provide a technology of further enhancing the in-plane uniformity of optical characteristics of an optical film, particularly, a halftone phase shift film formed in a blank (photomask blank) used for manufacturing a photomask.